Hydrogen-bonding patterns in a series of multi-component molecular solids formed by 2,3,5,6-tetramethylpyrazine with selected carboxylic acids

Abstract

The analysis of noncovalent interactions in several complexes constructed from 2,3,5,6-tetramethylpyrazine with different acid ligands, 1,4-cyclohexanedicarboxylic acid, 2,6-dihydroxybenzoic acid, 2,6-pyridinedicarboxylic acid, 6-hydroxy-2-naphthoic acid, 3-nitrophthalic acid, o-phthalic acid and 3-hydroxybenzoic acid, supported by single crystal X-ray diffraction analysis is presented. It reveals that all of these forms except 2 are organic supramolecular cocrystals without charge-transfer between the multicomponent acids and the base. The noncovalent interactions directing the assemblies of the eight structures are managed by classical O–H⋯O, O–H⋯N, weak C–H⋯O and π–π stacking interactions to generate 2D or 3D supermolecular architectures. For 5, 6, 7 and 8, carboxyl/pyrazine supramolecular heterosynthons R²₂(6) and R²₂(8) containing classical O–H⋯N and weak C–H⋯O interactions, usually observed in organic cocrystals of carboxylic acid and heterocyclic base, are again confirmed to participate in constructing these hydrogen-bonding supermolecular networks. In addition, weak C–H⋯O interactions were involved in building and consolidating their structures in all organic complexes. The thermal stability of crystals 1–8 has been investigated by thermogravimetric analysis (TGA) of mass loss.